Your kiddo just needs to put a bag on their leg and stand in the shower where the water won't run in it and wash a bit then get out. Go rent a shower chair for her to use for the next few weeks and have her sit on it and prop her leg on the edge of the shower in the corner or leave the door open a bit for her foot to stick out. She'll get the hang of it and soon be doing okay.
If it's the only shower in the house then perhaps she needs to go to a friends house that has a walk in shower a time or two per week then do quick/wash her hair sorts of showers at your house.
I had a full length cast on one time and it was fiberglass so I could get it wet. Best thing ever. I could stand up and get totally wet. Put a few drops of shampoo in my cast, swish it up and down, watch the bubbles come out of every hole, rinse it over and over until there were no more bubbles, then when I got out I could turn the blow dryer on, low-medium heat, and blow dry my cast. When it came off there was no yucky skin build up that smelled, no scratch marks from putting a hanger down the cast to scratch itching, or anything like that.
My friends girl got a waterproof cast on her leg and she was actually able to get in their pool in the shallower end and play every day. As long as it is able to have air around it and not mold or mildew they are wonderful.
I think your daughter will be fine but no tub bathes or standing showers where all the water will fill her non-waterproof cast can get ruined.